Gulf of Maine Seasonal Sea Surface Temperature Report

This report was created to track the seasonal patterns in sea surface temperature regimes for the Gulf of Maine as part of efforts by the Gulf of Maine Research Institute.

Satellite sea surface temperature data used was obtained from the National Center for Environmental Information (NCEI). With all maps and figures displaying NOAA’s Optimum Interpolation Sea Surface Temperature Data.

Gulf of Maine Extent

The spatial extent that we’ve used for any “regional” analyses or figures is displayed below. This bounding box is the same bounding box coordinates used to clip the OISST data when constructing the time series data from the array.

Weekly Summer Temperature Anomalies

Area-specific time series are the most basic building block for relaying temporal trends. For the Gulf of Maine (and other areas around the world!) we can generate a time series of the mean sea surface temperature within that area for each day.

By knowing how hot or cold it was in the past we can compare how observed temperatures compare with what the expected conditions are using a climatology from a specified reference period.

This section highlights how the surface ocean temperatures of recent weeks compare to the climatological average spanning 1982-2011.

Summer Temperature Record

Over the period of 1982-2020 the Gulf of Maine has warmed at a rate of 0.64\(\frac{^oC}{Decade}\), and more recently at a rate of 0.9\(\frac{^oC}{Decade}\) for the last decade (since 2010). While the rate of summer warming has slowed, the average temperature over the three-month period has not fallen below \(15^oC\) since 2007. Prior to 2007 this had only occurred seven times in 25 years (1982-2007).

Summer Temperature Anomaly Record

When looking at the temperature anomalies (departure from normal climate), we see the same pattern of rapid increase followed by a leveling off at temperatures \(1-2~^oC\) above what they were previously.

Summertemperatures since 2008 have not fallen below \(0.5~^oC\) with 2012 remaining the hottest summer at 2.3384736\(~^oC\) above the 30-year summer average.

Annual Trend

If we look at the pattern for year-round temperature we can see that the Gulf of Maine has been warming at a rate higher than the global average of \(0.15~^oC\) per decade. We can also see that that pause in summer temperature increase is less pronounced, an indication that other seasons are continuing to warm.

Marine Heatwaves

A marine heatwave is defined as a situation when seawater temperatures exceeds a seasonally-varying threshold (usually the 90th percentile) for at least 5 consecutive days. Successive heatwaves with gaps of 2 days or less are considered part of the same event. The heatwave threshold used below was 90%. Note, the climate reference period is 1982-2011, so the second figure really illustrates just how anomalously warm the most recent decade has been in a climatological context.

Over the course of 2021 the Gulf of Maine has been in heatwave conditions for much of the year. Beginning with a marine heatwave event in January that persisted through until march. Another brief event happened mid-march, before a third event which ended in the end of April. The longest heatwave of the year began at the very end of April, lasting until the end of July, peaking at nearly \(4~^oC\) above what we would have considered “normal” for that time of year. Now back in August we again are in heatwave conditions in the \(2-3~^oC\) range for temperature anomalies.

Changing the axis to anomalies rather than temperature the magnitude of the anomalies become more clear. The most severe marine heatwave event of this Summer occurred during the month of June, when temperatures rose above \(3.5~^oC\) above the climate average. Remember, the value of zero in this figure is the average daily temperature from the climate period of 1982-2011. Not a single day this year was within \(0.5~^oC\) within that average temperature.

Marine Heatwave Record

To more clearly see the progression away from the temperature regime of the 80’s and 90’s and into a the warmer norm we can use a heatmap plot. This figure presents each year as a row, with each day within the year colored in accordance to how hot or cold it was relative to the climatology we’ve used. In this figure heatwave events are also displayed to highlight periods of exceptional deviance from the climate average.

With this view its apparent that since around 1999 the Gulf of Maine now see’s a significant increase in the number and duration of marine heatwaves. Temperature anomalies have progressively increased and occurrences of temperatures below the climate average have become increasingly rare in recent years.

The Transition to a Warmer Climate

As we move further away from our baseline climate period it becomes clear to see both: (1.) A reduction in the amount of days that the temperature is below the climate average, and (2.) A simultaneous reduction in the amount of days that temperatures are within half a degree of that average. With temperatures for much of the last decade are above \(1~^oC\) the 30-year average from 1982-2011.

Seasonal Anomaly Map

Over the course of the Summer season the Gulf of Maine and surrounding areas experienced above average temperatures for much of the area, with particularly warm patches South and Southeast of Georges Banks. The peak daily temperature anomaly was 9.34\(^oC\), with a peak seasonal average of 5.09\(^oC\).

Warming Rate Ranking

If we look at the rates of change for each grid cell, rather than the observed temperature, its possible to rank how hot each location on earth is warming relative to others. By ranking those warming rates, and then taking the average ranking within the Gulf of Maine, we can obtain the average warming rank for our area compared to the rest of the globe.

Based on data from 1982-2020, the warming rates of Gulf of Maine have been some of the highest in the world. The area as a whole has been increasing at a rate of 0.048\(^{\circ}C/year\) which is faster than 96.2% of the world’s oceans.

Over that same period locations within the Gulf of Maine have been warming at rates as low as 0.014\(^{\circ}C/year\) and as rapidly as 0.097\(^{\circ}C/year\), corresponding to ranks as low as 50.8% and as high as greater than or equal to 99.5%.

 

A work by Adam A. Kemberling

Akemberling@gmri.org